The present invention relates generally to ophthalmic instruments, and more particularly to non-contact tonometers that measure intraocular pressure (IOP) by directing a fluid pulse at an eye to transfigure the cornea.
Non-contact tonometers are well-known in the field of ophthalmology for measuring intraocular pressure (IOP) by directing a fluid pulse at the cornea to cause observable deformation of the cornea. Most commonly, the observable deformation is a flattening of a predetermined area of the cornea, a condition known as applanation. In prior art non-contact tonometers, the fluid pulse is generated by a fluid pump system defining a plenum chamber for pressurized fluid. In order to direct the fluid pulse at the patient""s cornea, a narrow cylindrical fluid discharge tube is arranged in flow communication with the plenum chamber.
A common fluid pump system found in non-contact tonometers includes a rotary solenoid having a driven armature, a piston rod pivotally coupled to the solenoid armature, a piston fixed to the end of the piston rod for travel therewith, and a cylinder receiving the piston in close slidable fit to allow axial movement of the piston relative to the cylinder during a compression stroke. An example of this type of fluid pump system can be seen in U.S. Pat. No. 3,585,849 issued Jun. 22, 1971. In fluid pump systems of the type described, the driven armature, piston rod, and piston are connected in series, and thus the radial position of the piston in the cylinder is influenced by the driven armature and piston rod. Consequently, the solenoid, piston rod, piston and cylinder must be manufactured and located according to close tolerances to ensure that the piston moves in axial alignment with the cylinder during a compression stroke.
In more recent non-contact tonometers, linear motors are used to drive the piston. The piston is either directly coupled to an axially driven armature of the linear motor, or is indirectly coupled to the axially driven armature by a piston rod to which the piston is fixed. Here again, the components of the fluid pump system must be manufactured and mounted in the instrument according to close tolerances to ensure axial alignment of the piston with the cylinder for providing reliable and repeatable performance.
The necessity to design, manufacture, and assemble component parts of the fluid pump system of a non-contact tonometer pursuant to critical tolerance specifications adds to the production time and cost of the instrument. Despite time consuming and expensive efforts to provide a smooth and repeatable piston compression stroke, fluid pump systems of the prior art are accompanied by performance limitations caused by the interconnection of the various system components.
Therefore, it is an object of the present invention to provide a non-contact tonometer with an improved fluid pump system that is easier, less costly, and faster to design and manufacture.
It is another object of the present invention to provide a non-contact tonometer with a fluid pump system that exhibits improved performance over tonometer fluid pump systems of the prior art.
It is a further object of the present invention to provide a non-contact tonometer with an improved fluid pump system that involves relaxed tolerances in manufacture and assembly alignment, and which avoids the need for complex universal joint couplings.
The invention is embodied in a non-contact tonometer of a general type comprising a fluid pump system, a fluid discharge tube in communication with the fluid pump system for directing a fluid pulse at a patient""s eye to cause applanation of the cornea, applanation detection means for monitoring the cornea to detect applanation caused by the fluid pulse, means for determining a fluid pressure within a plenum chamber of the fluid pump system at a moment when the cornea reaches applanation, and processing means for correlating the plenum pressure with an intraocular pressure of the patient""s eye. In accordance with a preferred embodiment of the present invention, the fluid pump system comprises a linear solenoid having a driven member in the form of a plunger that moves axially when the solenoid is energized, and a piston situated adjacent an abutment end of the plunger for engagement thereby. The piston is not coupled to the plunger, but is merely pushed during a compression stroke to move axially relative to a surrounding cylinder with which the piston cooperates to define a compression chamber. A spring bearing against a front wall of the cylinder and a leading surface of the piston provides return motion to the piston. As will be appreciated, decoupling the piston from the solenoid plunger makes the axial alignment of the piston relative to the cylinder independent of the axial alignment of the plunger relative to the cylinder. In other words, the piston will remain aligned for slidable axial movement within the cylinder even if the plunger is xe2x80x9coff centerxe2x80x9d with respect to the cylinder and/or piston. Thus, the present invention allows for relaxation of tolerances and ensures that the piston remains in axial alignment with the cylinder over time for generating a more consistent air pulse to improve measurement performance.